Fiber optic networks use light signals to transmit data over a network. Although light signals are used to carry data, the light signals are typically converted into electrical signals in order to extract and use the data. The conversion of an optical signal into an electrical signal is often achieved utilizing a fiber optic receiver. A fiber optic receiver converts the optical signal received over the optical fiber into an electrical signal, amplifies the electrical signal, and converts the electrical signal into an electrical digital data stream.
The fiber optic receiver usually includes a photodiode that detects the light signal and converts the light signal into an electrical signal or current. A transimpedance amplifier amplifies the signal from the photodiode into a relatively large amplitude electrical signal. The amplified electrical signal is then converted into a digital data stream.
The optical signals that are converted into electrical signals by the fiber optic a receiver, however, can vary significantly in both amplitude and power. The power of the optical signal is often related, for example, to the length of the optical fiber over which the optical signal was received, the laser source, etc. These and other factors result in optical signals whose incident power at the transimpedance amplifier can vary significantly.
Fiber optic receivers are only able to successfully receive and amplify optical signals that fall within a particular power range. In order for a fiber optic receiver to accommodate a wide range of optical signals, the fiber optic receiver and in particular, the transimpedance amplifier (TIA), should be able to detect and amplify very low levels of optical power as well as high levels of optical power. The range of signals that can be successfully amplified is therefore effectively limited by the incident optical power because the fiber optic receiver distorts or clamps signals whose optical power is too large and cannot recognize signals whose optical power is too low due to noise limitations of the receiver.
In some applications, especially those applications at high optical power and high speed such as 10 Gigabits/second, it is undesirable for signal distortion or clamping to occur. For example, optical transceiver modules have begun to implement Electronic Dispersion Compensation (EDC) to extend the reach of transmission over legacy multimode fiber at high data rate. These multimode fibers can vary significantly in their frequency response and exhibit limited bandwidth and high dispersion. The 10GBASE-LRM standard is one of the first standards for fiber optic transceivers to use adaptive equalizers to compensate for non-ideal fiber channel frequency response. However, the equalization of a linear transmission channel and the implementation of EDC requires the TIA to be linear within a large incident optical power.
At low optical power, a TIA will usually operate in a linear fashion as there is not enough optical power to cause the transistors and other components of the transimpedance amplifier to saturate or otherwise cause non-linear signal distortion. At high optical power, however, saturation may occur with corresponding non-linear signal distortion.
Accordingly, several solutions have been implemented to provide for a linear TIA at high optical power and high speed. For example, referring to FIG. 1, one common solution is illustrated. FIG. 1 illustrates a TIA 10 coupled to a photodiode 15. Modulation current is received by a forward gain stage 11 and a shunt feedback resistor 12. The signal is then provided to a second gain stage 13 for further amplification. A feedback loop and amplifier 14 is used to sense an average photodiode current to determine the optical power received by photodiode 15. The shunt feedback resistor 11 may then be adjusted as needed to maintain the gain of TIA 10 in a linear fashion. However, the need to continually adjust shunt feedback resistor 11 as the average optical power changes can cause TIA 10 to become unstable, its bandwidth to increase and thus produce unwanted additional noise, peaking and potential non-linear signal distortion. Accordingly, the TIA of FIG. 1 requires special attention for high optical power application and is not a good candidate for linear amplification without special additional circuit implementation.
Another common solution is illustrated by FIG. 2. FIG. 2 illustrates a TIA 20 coupled to a photodiode 25. Modulation current is received by a forward gain stage 21 and a shunt feedback resistor 22. The signal is then provided to a second gain stage 23 for further amplification. A peak detector or full wave rectifier 24 is used to sense the amplitude of an output signal. The shunt feedback resistor 22 may then be adjusted to change the gain of the forward gain stage 21 such that the amplitude of the output signal stays constant. This is a typical implementation of an automatic gain control (AGC) circuit for a TIA. However, it is often difficult to implement a peak detector or full wave rectifier at speeds of 10 Gigabits/second or higher. Further, the need to change shunt feedback resistor 22 may cause the stability problems discussed in relation to FIG. 1. Finally, AGC does not necessarily guarantee the linearity of the amplifier. Accordingly, the simple TIA architecture of FIG. 2 is also not suited for linear optical to electrical conversion and amplification. Additional circuits may be required to meet linearity requirements.